The present invention relates to an ink jet printing apparatus of the type which ejects ink drops from an ink jet head and charges and deflects them to print out data on a recording sheet. More particularly, the present invention is concerned with an ink jet printing apparatus which compensates for print distortions and thereby enhance high quality data reproduction by interpolating guard drops between adjacent printing drops which are adapted to compensate for the influence of a charge deposited on an immediately preceding printing drop.
In an ink jet printer of the type described, charged ink drops fly adjacent to each other so that the air resistance, Coulomb's force and like factors may effect the spacing between the adjacent ink drops or the immediately preceding charged drop may reduce a charge on a following drop which is about to be charged. This would disturb the flight of the drops to develop print distortions on a recording medium and thereby prevent reproduction of clear-cut images. Thus, implementations have been proposed for compensating the flight of ink drops. While the compensation may be achieved most easily by shortening the distance between the ink jet head and the recording medium, such requires an increase in the amount of charge to be deposited on printing drops. An increase in the charge, however, would cause the charged printing drop to charge a succeeding uncharged printing drop to the opposite polarity, again resulting in print distortions.
As has been proposed in Japanese patent publication No. 42334/1977, for example, the print distortions may be coped with by inserting a guard drop between adjacent printing drops for compensation purpose. This expedient still suffers from the following drawbacks which were confirmed by experiments. The guard drop between two adjacent printing drops becomes charged by preceding one of the printing drops to a polarity opposite to the latter and, due to the resulting Coulomb's force therebetween, the guard drop is caused to join the printing drop. Also, the adjacent printing drops tend to join each other because attraction acting between the guard drop and the preceding printing drop serves to shorten the distance between the printing drops, or they tend to more intensely effect each other, if not join each other, to thereby noticeably degrade the reproduced data. Such a tendency is particularly pronounced in a high deflection step range, in which the amount of charge is significant.
The present invention contemplates to overcome the problem discussed above by suitably determining the number of guard drops interpolated between printing drops and a charging level for the guard drops. Although the patent publication mentioned describes a technique for charging guard drops, it is nothing more than giving guard drops a charge higher than those on printing drops in order to suppress charge repulsion between adjacent printing drops. That is, it does not even suggest a unique charging system for guard drops in accordance with the present invention which also takes into account the attraction between a charging ink drop and guard drops.